When a microprocessor boots, it accesses a boot configuration in order to retrieve configuration values. This is done through a boot interface on the microprocessor. One of the most common boot interfaces is an I2C interface. For a microprocessor supporting an I2C interface, the boot interface accesses two pins leading from the microprocessor. The microprocessor may support other interfaces, in which a different number of dedicated pins are used, or in which a bus and address are used.
Typically, the boot configuration is stored in a PROM, and the two pins through which the I2C interface communicates lead to the PROM. The PROM is a dedicated device within a socket on the same board (or card) as the microprocessor. If the boot configuration is to be reconfigured, the PROM can be removed from the socket and reprogrammed in a reprogramming station. Field management of such PROMs is extremely difficult, as the reprogramming stations are usually at a central location.
Some PROMs are in-circuit reconfigurable. Such PROMs need not be removed from their socket in order to be reprogrammed. Reprogramming is often done by a field technician through a physical connector on the board that leads the PROM. The physical connector and its connections leading to the PROM occupy real estate on the board.
Some in-circuit reconfigurable PROMs can be accessed by other processors for reconfiguration. While this removes the need for a separate physical connector, additional signaling to and from the PROM must be used. And in any event, the dedicated PROM and socket still occupy real estate on the board.